ndisc: accept unknown ip6-privacy value
Summary
The value can be unknown for different reasons:
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we don't have a value saved in NMDevice's "ip6_saved_properties" because NM was restarted or because the device didn't have an ifindex when it became managed.
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the value read from /proc is outside the allowed range (kernel allows "echo 42 > /proc/sys/net/ipv6/conf/enp1s0/use_tempaddr")
Note that the second case was also possible before commit 797f3caf ('device: fall back to saved use_tempaddr value instead of rereading /proc').
If we can't determine the previous value, pass "unknown" to ndisc; it will generate a l3cd with "unknown" ip6-privacy, which means to not set the value when committing the configuration.
Fixes: 797f3caf ('device: fall back to saved use_tempaddr value instead of rereading /proc')
Checklist
Please read https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/CONTRIBUTING.md before opening the merge request. In particular, check that:
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the subject for all commits is concise and explicative -
the message for all commits explains the reason for the change -
the source is properly formatted -
any relevant documentation is up to date -
you have added unit tests if applicable -
the NEWS file is updated when the change deserves to be mentioned, for example for new features, behavior changes, API deprecations, etc.